Power converter arrangements are used in many applications, among other things for generator and motor drives. A power converter arrangement usually comprises a power converter whose direct current (DC) side is connected to a DC intermediate circuit whose alternating current (AC) side is connected with an AC voltage grid and/or an electrical load, such as, for example, a rotating electrical machine or a transformer, and that has controllable switching elements that can be controllably switched at a high-frequency to convert the DC voltage of the DC intermediate circuit into a multiphase AC voltage to feed the AC side. A controller controls the operation of the power converter in a way that is suitable for the application.
High power variable speed asynchronous machines, such as, for example, doubly fed induction machines (DFIM), are frequently controlled from the rotor side to reduce the rated power of the electronic power converter. The power converter is normally used to control the active power and reactive power of the stator of the asynchronous machine indirectly through control of the rotor current. Such systems have a large advantage over so-called full power conversion systems with an AC/DC/AC converter on the stator side, since only the slip power of the machine must be handled by the power converter. Thus, the rotor-side power converter can be dimensioned for only a fraction of the rated power of the machine, usually less than 25% of the rated power, depending on the rotor speed range. In transient situations caused, for example, by strong perturbations in the stator voltages, such as, for example, if there are voltage dips in the line voltage, short circuits, and the like, the voltage induced in the rotor can reach very high values in comparison with the normal rotor voltage that is induced at a given rotor slip. Accordingly, the rotor-side power converter can be exposed to very high surge rotor current transients that can considerably exceed the steady state values and nominal load capacity of the power converter switches. If the DC bus voltage or the rotor currents exceed certain safety limits, switching of the power converter switches must be prevented and overvoltage protection, so-called crowbar, must be activated. The protective crowbar effectively short circuits the rotor terminals, so that the currents in the power converter can quickly be reduced to zero. The protective crowbar is frequently constructed using a three-phase thyristor bridge, switches with anti-parallel thyristors or diodes, or a three-phase diode bridge and a single thyristor on the DC side. In case the overvoltage protection is turned on, it normally remains activated until the stator is separated from the grid.
To satisfy today's grid connection requirements, it is necessary to restore normal power converter operation as soon as possible. One option for achieving this is to interrupt normal power converter switching temporarily if high transient currents exceed a predetermined limit, and allow the rotor currents to continue to flow through the freewheeling diodes and charge the intermediate circuit capacitors on the DC bus. The DC intermediate circuit voltage of the power converter can then be controlled by a brake chopper that dissipates the excess energy out of the intermediate circuit to a brake resistor transforming it into heat therein. This can prevent the intermediate circuit voltage from rising to an unallowable value and destroying the intermediate circuit capacitors or other components of the circuit. Consequently, an additional resistor is effectively inserted into the rotor circuit, which may produce various advantages, such as, for example, a reduction of the rotor transient currents, an improved power factor of the rotor (increased torque production), and faster decay of the aperiodic components of the rotor transient current (smaller rotor time constant). Once the rotor surge current is reduced, the brake choppers are turned off, and the normal switching of the power converter can be restored. This allows the rotor current control to be resumed with minimal delay after the transient event.
The major disadvantage of this solution is that the freewheeling diodes of the power converter and the brake choppers must be greatly over-rated to cope with the transient rotor surge currents. The freewheeling diodes and the brake choppers must frequently be paralleled to provide the necessary surge current ratings. An alternative solution that does not require an increase of the surge current rating of the power converter involves adding, in parallel with the power converter, an additional diode bridge that is equipped with a brake chopper and a resistor. Once the current or the DC bus voltage of the power converter exceeds its limit, the external brake chopper can be activated to absorb part of the rotor current, relieving the power converter. Functionally, there is no essential difference between the brake choppers that are provided inside or outside the power converter.
Taking into consideration that the paralleled external bridge is used relatively rarely and only during stator-side transients producing excessive rotor currents that the power converter itself cannot cope with, in high-power applications it is technically and economically justifiable to replace the external diode bridge and the brake chopper by a thyristor bridge or a diode bridge and a thyristor switch. In this case, the turning on of the external brake resistor is controlled by firing the thyristors or thyristor of the external bridge. Unfortunately, once the thyristors are activated they cannot be turned off until the rotor currents naturally or forcedly fall to zero. Since the rotor transient current can contain both DC and low-frequency components, the rotor currents might not have any zero crossing for a prolonged period of time, and the thyristor turn-off time cannot be precisely controlled or guaranteed. This could be potentially problematic, as restoring normal power converter operation might be delayed due to the inability to deactivate the external brake bridge immediately after the rotor transient currents have fallen sufficiently low so that normal operation and machine current control could be resumed. Thus, it is important to provide a method that will reliably force turn off of the thyristors when normal power converter operation can be restored.
It is known to utilize the fully controllable switches of the power converter bridge to assist in turning off thyristors in the external rectifier. For example, known in the field is a protective device and a protection method for a power converter device that has several controllable switches, the protective device having an external protection circuit which is connected to the AC side of the power converter device and which comprises a three-phase rectifier bridge implemented by diodes, and a series circuit of a protective switch and auxiliary commutation means that comprises a plurality of diodes coupled in series. The series circuit is connected between the positive and the negative pole of the rectifier bridge, and the protective switch is a thyristor. Upon detection of an error in the circuit that exceeds certain conditions the protective device opens the power converter switches and triggers the thyristor protective switch to close it. This effectively short-circuits the rotor circuit, so that a short circuit current flows from the rotor, through the protective switch, and to the protection circuit. As soon as the protective device detects that the error condition has ended, it closes all three lower switches of the three-phase power converter circuit, which short-circuits all three phases of the AC voltage and connects them with the negative bus rail of the DC intermediate circuit. In space vector modulation, which is commonly used for controlling electrical rotating machines based on pulse width modulation (PWM), the latter step corresponds to the application of a zero voltage vector or passive voltage vector by the power converter, since no line-to-line voltage is measurable between the phases of the AC voltage. This is in contrast to active voltage vectors or non-zero voltage vectors that are output in other breaker switch positions, which then result in line-to-line voltages between the phases that are different from zero. Thus, the use of a zero voltage vector by the power converter essentially short-circuits the external rectifier bridge, wherein the power converter practically takes over the entire rotor current, after which turning off of the thyristor protective switch is ensured by discharging an additional capacitor which is connected in parallel to the auxiliary commutation device.
Also known in the art is a converter system and a method to operate a converter system for switching at least three voltage levels, wherein a by pass circuit comprising external brake resistors supplied via a thyristor bridge is provided. In case of a fault in which the current through the converter unit exceeds a predetermined threshold value, the thyristors are turned on by the application of a turn-on signal, so that the AC side of the converter unit is bypassed by the external resistors. The external thyristor bridge is turned off in two steps: First, a zero voltage vector is applied by the converter, effectively short-circuiting the external thyristor bridge, to redirect the current to the converter and substantially reduce the thyristor currents. Then, by closing two auxiliary power switches the thyristor bridge is connected (via auxiliary resistors) to the DC intermediate circuit and turned off by applying inverse voltage across the thyristors.
In the prior art, when the thyristors are turned off the power converter is used in a passive manner to relieve the external rectifier from the current, producing a short circuit, while the actual thyristor switch turn off is ensured by additional means, such as, for example, by an additional DC-side capacitor or auxiliary switches. This increases the expense both for the circuitry of the protective device and also for the protection method. There is a need to reduce this expense.
It is an object of the invention to eliminate the shortcomings of the prior art and to provide an improved method and an improved device to protect a power converter arrangement. In particular, it is an object of present invention to provide a method to protect a power converter arrangement and a power converter arrangement with a protective device, which allow deactivation of the protective device with a small delay and with reduced expense.